[kubatyszko]

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My recently acquired Indigo2 with a genuine SGI monitor (including SGI support sticker ;-)), works over VGA to my LCD monitor, but when it’s hooked up to the CRT the picture has strange distortion around bottom 1/3 of the picture, it also flickers a little.
What does the collective wisdom say? Capacitors ?

Update - turns out that posting this from my phone did not upload the screenshot doh...

   

[robespierre]

No information whatsoever. Have you checked that the monitor supports the resolution and frequency of the graphics mode you are using?

SGI never made any monitors. The OEMs were Hitachi, Mitsubishi, or Sony. Diagnosis and repair will depend on the chassis design: for example, fixed-frequency sets don't use digital controls, but later multisync ones do.

If there is a distortion in the lower half of the display only, it points to a vertical sweep problem, perhaps one polarity of drive to the vertical yoke is bad. Could be a transistor in the sweep circuit, or a capacitor like you said. But not necessarily an electrolytic.

[chulofiasco]

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My recently acquired Indigo2 with a genuine SGI monitor (including SGI support sticker ;-)), works over VGA to my LCD monitor, but when it’s hooked up to the CRT the picture has strange distortion around bottom 1/3 of the picture, it also flickers a little.
What does the collective wisdom say? Capacitors ?

Update - turns out that posting this from my phone did not upload the screenshot doh...

See if you can find the OEM tech pub for the tube.. to me it needs to be adjusted.  Probably a combination of caps and pots.

[kubatyszko]

Note to self, don't post stuff over the phone, the photo didn't get included.

Yeah, this is a Sony OEM monitor and the service manual is easy to gind.
The monitor definitely supports the resolution, except the bottom 1/3 distortion.

Thanks

[robespierre]

Looks like a GDM-20E21. The good news is that the picture looks fine apart from some geometric distortion. That model uses digital calibration, no pots to be found.
Getting Sony DAS running to edit the calibration is likely the next step. Looks like this post has some information on how to do that.

[kubatyszko]

Good and bad news.

Good:

I got DAS working (wow that software is so archaic). Luckily the ECS port is under a flap on the back, easy to access. I literally had to find an IDE drive, install WinXP on an old PC for that.
Ironically, nowhere on the Internet there's any mention of serial port specifics used for DAS - it MUST be serial COM2  (even if I used USB converter, it needs to be reassigned as COM2 in the device manager).
My monitor is a GDM-4011P which according to service manual (I do have a matching one) is based on N3 chassis.
I was able to communicate with it using DAS J4.6.5 and selecting GDM-400PS (GDM-500PS also works).
I can't "upload" the EEPROM to save a backup though, during the process DAS is asking to hook up a test pattern generator which I don't have, so that's okay for now.

   

Bad (but not really):

I'm able to communicate with the monitor, and the bad news, is that on the "fault information" page everything looks good - so that might mean that the distortion is of an electronic nature, perhaps caps (which wouldn't be that terrible).

   


Yeah, so the next step is to get a pattern generator that can actually output the mode required (1600x1200) and see if I can get through the round of adjustments.
Still, I'm getting mentally ready to take the monitor apart and go through the pain of replacing capacitors :/

[kubatyszko]

Oh joy, I mostly solved the issue. While I was playing around with DAS, I also realized that the monitor was on a shelf directly under two rackmount servers that I actively use.
Somehow it struck me that maybe, just maybe I have some interference (also knowing that my power in the garage isn't that great, poor grounding etc).
I may have been inspired by my old knowledge about speakers that detected incoming phone calls, and some being strong enough to distort picture on CRT's.
Yeah, I moved the monitor away, currently on a chair and the distortion mostly went away, it's barely noticeable and very sporadic.
It's still possible the monitor may have components that are aging which makes it more sensitive to power or interference, but for now I mostly solved my issue.
Next up, fix the power in the garage.

[robespierre]

Interesting! It's clearly the case that magnetic fields from speaker magnets will distort CRTs (any magnetic field that is strong enough over a large enough area of space) that are not magnetically shielded, which is most CRTs.
Looks like you discovered that the rack servers' power supplies (or maybe fans or disks?) have strong magnetic fields.
There are some devices that can measure the fields (one example is the F.W. Bell 4070) to quantify how much interference the servers are putting out.

[vishnu]

Interesting! It's clearly the case that magnetic fields from speaker magnets will distort CRTs (any magnetic field that is strong enough over a large enough area of space) that are not magnetically shielded, which is most CRTs.
Looks like you discovered that the rack servers' power supplies (or maybe fans or disks?) have strong magnetic fields.
There are some devices that can measure the fields (one example is the F.W. Bell 4070) to quantify how much interference the servers are putting out.

The short answer is that it's impossible to block/sheild the magnetic field. If you're familiar at all with Maxwell's equations, the divergence of the B (magnetic) field is zero. The only thing you can do is move the magnets further away from whatever device they're dorking with.